1Combine the mayonnaise, whole grain mustard, fresh thyme leaves, and a pinch of salt to make the burger sauce and place in the fridge to develop.
2Heat a heavy bottom pan over medium heat and add olive oil. Add the diced shallots and fresno chiles, sautéing for 3-5 minutes until softened.
3Add the halved cherry tomatoes, season with salt and pepper, and cook for 5-7 minutes until they break down.
4Smash the tomatoes with a wooden spoon, add the sherry vinegar, and stew for 5-10 minutes until thickened.
5Remove from heat, stir in the fresh basil, and set aside to cool.
6Smash the beef balls into a hot griddle on medium-high heat, season, and cook for at least 90 seconds. Flip, top with cheese, and remove once the cheese melts. Toast the buns.
7Assemble the burger by spreading sauce on the bottom bun, adding arugula, two patties, a scoop of tomato relish, and the top bun with more sauce.